Liquid-crystalline medium and liquid-crystal display

ABSTRACT

The invention relates to a liquid-crystalline medium of positive dielectric anisotropy, characterized in that it comprises in each case one or more compounds of the general formulae I, II and IV                    
     in which the parameters are as defined in the text, and to the use thereof as a constituent of a multibottle system and in electro-optical displays, and to multibottle systems of this type and displays which contain this medium.

The present invention relates to a liquid-crystalline medium, inparticular one of low optical anisotropy, and to the use thereof forelectro-optical purposes, and to electro-optical displays containingthis medium.

Liquid-crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (superbirefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic (TN) structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Further-more, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

For a low dependence of the contrast on the viewing angle, theobservance of a low optical retardation, as described in DE 30 22 818,preferably in the range from 0.4 to 0.5 μm, is advantageous in TN cells.

They must furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and, in particular on use in a display addressed by means ofa matrix of active switching elements, low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and low vapor pressure are desired.

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, electrically non-linear elements as activeelements (these can be, for example, transistors). The term “activematrix” is then used, where a distinction can be made between two types:

1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer assubstrate.

2. Thin-film transistors (TFTS) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. The latter technology is being worked on intensivelyworldwide.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully color-capabledisplays, in which a mosaic of red, green and blue filters is generallyarranged in such a way that a filter element is opposite each switchablepixel.

The TFT displays usually operate as TN cells with crossed polarizers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display with integratednon-linear elements, i.e., besides the active matrix, also displays withpassive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TVs) or for high-information displays for computerapplications (laptops) and in automobile or aircraft construction.Besides problems regarding the angle dependence of the contrast and theresponse times, difficulties also arise in MLC displays due toinsufficiently high specific resistance of the liquid-crystal mixtures[TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p.141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Designof Thin Film Transistors for Matrix Addressing of Television LiquidCrystal Displays, p. 145 ff, Paris]. With decreasing resistance, thecontrast of an MLC display deteriorates, and the problem of after-imageelimination may occur. Since the specific resistance of theliquid-crystal mixture generally drops over the life of an MLC displayowing to interaction with the interior surfaces of the display, a high(initial) resistance is very important in order to obtain acceptableservice lives. In particular in the case of low-volt mixtures, it washitherto impossible to achieve very high specific resistance values. Itis furthermore important that the specific resistance exhibits thesmallest possible increase with increasing temperature and after heatingand/or UV exposure. The low-temperature properties of the mixtures fromthe prior art are also particularly disadvantageous. It is demanded thatno crystallisation and/or smectic phases occur, even at lowtemperatures, and the temperature dependence of the viscosity is as lowas possible. The MLC displays from the prior art thus do not meettoday's requirements.

Besides liquid-crystal displays which use back lighting, i.e. areoperated transmissively and optionally transflectively, there is alsoparticular interest in reflective liquid-crystal displays. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than back-litliquid-crystal displays of corresponding size and resolution. Since theTN effect is characterised by very good contrast, reflective displays ofthis type are readily legible even under bright ambient conditions. Thisis already known of simple reflective TN displays, as used, for example,in wristwatches and pocket calculators. However, the principle can alsobe applied to high-quality, higher-resolution active matrix-addresseddisplays, such as, for example, TFT displays. Here, as is already thecase in the generally conventional transmissive TFT-TN displays, the useof liquid crystals of low birefringence (Δn) is necessary in order toachieve low optical retardation (d·Δn). This low optical retardationresults in a low viewing-angle dependence of the contrast, which isusually acceptable (cf. DE 30 22 818). In reflective displays, the useof liquid crystals of low birefringence is much more important than intransmissive displays, since in reflective displays, the effective layerthickness through which the light passes is approximately twice as greatas in transmissive displays of the same layer thickness.

There thus continues to be a great demand for MLC displays and inparticular reflective MLC displays having very high specific resistanceat the same time as a large working-temperature range, short responsetimes even at low temperatures and low threshold voltage which do nothave these disadvantages, or only do so to a reduced extent.

Easy matching of the properties of the liquid-crystal mixture to therequirements of specific liquid-crystal cells of the respective displaytypes is desired. In order to achieve this requirement, so-calledmultibottle systems are employed. Multibottle systems of this typeconsist of individual liquid-crystal media which are to be mixed withone another. In the simplest case, they consist of two different mediawhich differ only in the value of one physical property, but haveessentially the same values in all others. Thus, for example, thevariable property may be the birefringence of the liquid-crystal medium,which enables matching of the optical retardation of the display cell toits layer thickness, or the threshold voltage of the medium, whichenables matching to the operating voltage available for addressing.4-bottle systems consist of four liquid-crystal media which differ ineach case in pairs in one of two properties and agree in all otherproperties.

DE 43 37 439 describes multibottle systems by means of whichliquid-crystalline compositions for TN displays having birefringencevalues in the range from 0.115 to 0.165 and threshold voltages of from1.2 to 2.1 V can be achieved. However, these liquid-crystal media havelow clearing points (below 70° C.) and low voltage holding ratio valueswhich are not adequate for most AMD applications. DE 196 03 257describes multibottle systems which cover a similar parameter range. Thebirefringence extends from 0.120 to 0.160 and the threshold voltage from1.2 to 2.2 V. The clearing points are significantly higher than those ofthe systems in DE 43 47 439, but the voltage holding ratio values areeven lower.

In TN (Schadt-Helfrich) cells, media are desired which facilitate thefollowing advantages in the cells:

broadened nematic phase range (in particular down to low temperatures)

the ability to switch rapidly at low temperatures,

increased stability to UV radiation (longer life)

lower threshold (addressing) voltage, and

low birefringence, particularly for an improved viewing angle range.

The media available from the prior art do not enable these advantages tobe achieved while simultaneously retaining the other parameters.

The invention has an object of providing media, in particular for MLCdisplays of this type, which do not have the above-mentioneddisadvantages, or only do so to a lesser extent, and preferably at thesame time have very high specific resistances and low threshold voltagesas well as low birefringence values.

It has now been found that this and other objects can be achieved ifmedia according to the invention are used in displays.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

The present invention furthermore has an object of providing multibottlesystems and in particular 4-bottle systems which conform to therequirements of AMD applications.

This is achieved by the use of media according to the invention.

The invention thus relates to a liquid-crystalline medium based on amixture of polar compounds of positive dielectric anisotropy,characterised in that it comprises one, two or more compound(s) of thegeneral formula I

in which

R¹ is an alkyl or alkenyl radical having 1 or 2 to 7 carbon atomsrespectively, preferably having 2 to 5 carbon atoms, preferably astraight-chain radical, particularly preferably an alkyl radical, and

X¹ is F, OCF₃ or OCHF₂, and

one, two or more compound(s) of the general formula II

in which

R² is an alkyl or alkenyl radical having 1 or 2 to 7 carbon atomsrespectively, preferably having 2 to 5 carbon atoms, preferably astraight-chain radical, particularly preferably an alkyl radical, and

X² is F, OCF₃ or OCHF₂, and

one or more compound(s) of the general formula IV

in which

R⁴ is an alkyl or alkenyl radical having 1 or 2 to 7 carbon atomsrespectively, preferably having 2 to 5 carbon atoms, preferably astraight-chain radical, particularly preferably an alkyl radical,

X⁴ is F, Cl, OCF₃ or OCHF₂, preferably F or OCF₃, and

k is 0 or 1, and

optionally one or more compound(s) of the general formula III

in which

R³ is an alkyl or alkenyl radical having 1 or 2 to 7 carbon atomsrespectively, preferably having 2 to 5 carbon atoms, preferably astraight-chain radical, particularly preferably an alkyl radical,

Z³² and, if present, Z³¹ are each, independently of one another,—CH₂—CH₂—, —CH═CH— or a single bond, preferably only one of Z³¹ and Z³²is —CH₂—CH₂— or —CH═CH—,

preferably

particularly preferably

X³ is F, OCF₃ or OCHF₂, preferably F, and

r is 0 or 1,

and optionally one or more compound(s) of the general formula V

in which

are each, independently of one another,

preferably

particularly preferably

R⁵¹ and R⁵² are each, independently of one another, an alkyl, alkoxy oralkenyl radical having 1 or 2 to 7 carbon atoms respectively, preferablyhaving 1 to 5 carbon atoms, preferably a straight-chain radical,particularly preferably an alkyl radical, and

n and m are each, independently of one another, 0 or 1.

The invention furthermore relates to a multibottle system, preferably a2- or 4-bottle system, particularly preferably a 4-bottle system, bymeans of which selected properties of a liquid-crystal medium can beadjusted within a prespecified range by simple mixing. Preferably thebirefringence or the threshold voltage, particularly preferably bothproperties, can be adjusted.

The media according to the invention preferably comprise one or morecompounds of the formula I selected from the group consisting of thecompounds of the formulae Ia to Ic

in which

R¹ is an alkyl or alkenyl radical having 1 or 2 to 7 carbon atomsrespectively, preferably having 2 to 5 carbon atoms, preferably astraight-chain radical, particularly preferably an alkyl radical.

The media according to the invention preferably comprise one or morecompounds of the formula 11 selected from the group consisting of thecompounds of the formulae IIa to IIc:

in which

R² is an alkyl or alkenyl radical having 1 or 2 to 7 carbon atomsrespectively, preferably having 2 to 5 carbon atoms, preferably astraight-chain radical, particularly preferably an alkyl radical.

The media according to the invention preferably comprise one or morecompounds of the formula IIII selected from the group consisting of thecompounds of the formulae IIIa to IIII:

in which

R³ is an alkyl or alkenyl radical having 1 or 2 to 7 carbon atomsrespectively, preferably having 2 to 5 carbon atoms, preferably astraight-chain radical, particularly preferably an alkyl radical.

The media according to the invention preferably comprise one or morecompounds of the formula IV selected from the group consisting of thecompounds of the formulae IVa to IVc:

in which

R⁴ is an alkyl or alkenyl radical having 1 or 2 to 7 carbon atomsrespectively, preferably having 2 to 5 carbon atoms, preferably astraight-chain radical, particularly preferably an alkyl radical.

The media according to the invention preferably comprise one or morecompounds of the formula V selected from the group consisting of thecompounds of the formulae Va to Vh:

in which

R⁵¹ and R⁵² are each, independently of one another, an alkyl, alkoxy oralkenyl radical having 1 or 2 to 7 carbon atoms respectively, preferablyhaving 1 to 5 carbon atoms, preferably a straight-chain radical,particularly preferably an alkyl radical.

The compounds of the formulae I to V and their sub-formulae which can beused in the media according to the invention are either known or areprepared analogously to the known compounds by known methods.

The invention also relates to electro-optical displays (in particularMLC displays having two plane-parallel outer plates, which, togetherwith a frame, form a cell, integrated non-linear elements for switchingindividual pixels on the outer plates, and a nematic liquid-crystalmixture of positive dielectric anisotropy and high specific resistancewhich is located in the cell) which contain media of this type, and tothe use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention enable asignificant widening of the available parameter latitude.

The achievable combinations of clearing point, viscosity at lowtemperature, thermal and UV stability, optical anisotropy (i.e.birefringence) and threshold voltage are far superior to previousmaterials from the prior art.

The liquid-crystal mixtures according to the invention, while retainingthe nematic phase down to −20° C. and preferably down to −30° C.,particularly preferably down to −40° C., enable clearing points above75° C., preferably above 80° C., particularly preferably above 90° C.,simultaneously birefringence values ≦0.090 or ≧0.100, preferably ≦0.085or ≧0.105, particularly preferably ≦0.080 or ≧0.110, and a low thresholdvoltage to be achieved, enabling excellent MLC displays to be obtained.

In particular, the mixtures are characterised by low operating voltages.The TN thresholds are below 1.6 V, preferably below 1.5 V, particularlypreferably <1.4 V.

The mixtures according to the invention are particularly preferablycharacterised by a clearing point of 85° C. or above and

a threshold voltage of 1.50 V or less and a Δn of 0.085 or less or of0.120 or more and preferably

a threshold voltage of 1.40 V or less and a Δn of 0.090 or less or of0.11 0 or more.

It is evident to the person skilled in the art that, through a suitablechoice of the components of the mixtures according to the invention, itis also possible for higher clearing points (for example above 100° C.)to be achieved at lower dielectric anisotropy values and thus higherthreshold voltages or for lower clearing points to be achieved at higherdielectric anisotropy values (of, for example, 12 or more) and thuslower threshold voltages (for example <1.1 V) with retention of theother advantageous properties. Also at viscosities correspondinglyincreased only slightly, it is likewise possible to obtain mixtureshaving greater Δ∈ and thus lower thresholds. The MLC displays accordingto the invention preferably operate at the first Gooch and Tarrytransmission minimum [C. H. Gooch and H. A. Tarry, Electron. Lett. 10,2-4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584,1975], where, besides particularly favorable electro-optical properties,such as, for example, high steepness of the characteristic line and lowangle dependence of the contrast (German Patent 30 22 818), a lowerdielectric anisotropy is sufficient at the same threshold voltage as inan analogous display at the second minimum. This enables significantlyhigher specific resistances to be achieved using the mixtures accordingto the invention at the first minimum than in the case of mixturescomprising cyano compounds, in particular comprising benzonitrilecompounds. By selecting the individual components and their proportionsby weight, the person skilled in the art is able to set thebirefringence necessary for a pre-specified layer thickness of the MLCdisplay using simple routine methods. The requirements of reflective MLCdisplays have been described, for example, in Digest of TechnicalPapers, SID Symposium, 1998.

The rotational viscosity γ₁ at 20° C. is preferably <160 mpa.s,particularly preferably <130 mPa.s. The nematic phase range preferablyhas a width of at least 90 degrees, in particular at least 100 degrees.This range preferably extends from at least −30° C. to +80° C.,particularly preferably at least from −30° C. to +85° C. or at leastfrom −40° C. to 85° C.

Measurements of the capacity holding ratio, also known as the voltageholding ratio (also abbreviated to HR) [S. Matsumoto et al., LiquidCrystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference, SanFrancisco, June 1984, p. 304 (1984); G. Weber et al., Liquid Crystals 5,1381 (1989)] have shown that mixtures according to the inventioncomprising compounds of the formula I have an adequate HR for MLCdisplays. C denotes a crystalline phase, S a smectic phase, S_(C) asmectic C phase, S_(B) a smectic B phase, N a nematic phase and I theisotropic phase.

The media according to the invention preferably comprise a plurality of(preferably 2 or more) compounds of the formula I, and the proportion ofthese compounds is 5% -95%, preferably 10% -70% and particularlypreferably in the range 15% -65%.

The media according to the invention preferably comprise compounds ofthe formulae I to V in the concentrations given in the following table(Table 1).

TABLE 1 Preferred concentrations of the compounds Concentration rangec_(min)-c_(max)/% Formula preferably particularly pref. I 3-65 5-60 8-65II 3-40 4-30 5-25 III 2-50 3-45 4-40 IV 10-50  15-45  20-40  V 0-30 0-250-20

In a preferred embodiment, in particular in the case of the achievementof a “corner mixture” of a multibottle system of high birefringence, themedium preferably comprises compounds of the formulae I to V in theconcentrations given in Table 2.

TABLE 2 Preferred concentrations for media of high Δn Concentrationrange c_(min)-c_(max)/% Formula preferably particularly pref. I 3-305-25 5-20 II 3-40 4-30 5-25 III 2-50 5-45 10-45  IV 10-50  15-45  20-40 V 0-30 0-25 0-25

In a further preferred embodiment, in particular in the case of theachievement of a “corner mixture” of a multibottle system of lowbirefringence, the medium preferably comprises compounds of the formulaeI to V in the concentrations given in Table 3.

TABLE 3 Preferred concentrations for media of low Δn Concentration rangec_(min)-c_(max)/% Formula preferably particularly pref. I 10-65  20-60 25-55  II 3-40 4-30 5-25 III 0-50 2-35 3-25 IV 10-50  15-45  20-40  V0-30 0-20 0-10

Preferred embodiments are indicated below.

a) The medium comprises one or more compound(s) of the formula III.

b) The medium comprises one or more compound(s) selected from the groupconsisting of the compounds of the formulae Ia to Ic.

c) The medium comprises one or more compound(s) of the formula Ia.

d) The medium comprises one or more compound(s) of the formula Ib.

e) The medium comprises one or more compound(s) of the formula II inwhich X² is F.

f) The medium comprises one or more compound(s) selected from the groupconsisting of the compounds of the formulae IIIa to IIIb, preferablyselected from the group consisting of the compounds of the formulaeIIIa, IIIb, IIId, IIIg, IIIi, IIIj and IIIk, very particularlypreferably selected from the group consisting of the compounds of theformulae IIIa, IIIb, IIId and IIIg.

g) The medium comprises one or more compounds of the formula IVa or ofthe formula IVc, preferably of the formula IVa and IVc.

h) The medium comprises one or more compound(s) selected from the groupconsisting of the compounds Va to Vc, preferably of the formula Va orVc.

i) The medium comprises one or more compound(s) selected from the groupconsisting of the compounds Vd to Vf, preferably of the formula Vf.

j) The medium comprises one or more compound(s) of the formula Vg and/orof the formula Vh, preferably of the formula Vh.

k) The medium essentially consists of compounds of the formulae I to V,where “essentially” in this application means to an extent of greaterthan 50%, preferably to an extent of 80% or more and particularlypreferably to an extent of 90% or more.

l) The medium essentially consists of compounds of the formulae I to IV.

It has been found that even a relatively small proportion of compoundsof the formulae I and II mixed with conventional liquid-crystalmaterials, but in particular with one or more compounds of the formulaeIII and/or IV and/or V, results in a significant increase in the voltageholding ratio values, with broad nematic phases with low smectic-nematictransition temperatures being observed at the same time, improving theshelf life.

The term “alkyl” preferably covers straight-chain and branched alkylgroups having 1-7 carbon atoms, in particular the straight-chain groupsmethyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having2-5 carbon atoms are generally preferred.

The term “alkenyl” preferably covers straight-chain and branched alkenylgroups having 2-7 carbon atoms, in particular the straight-chain groups.Particularly preferred alkenyl groups are C₂-C₇-1 E-alkenyl,C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, inparticular C₂-C₇-1 E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl.Examples of further preferred alkenyl groups are vinyl, 1E-propenyl,1E-butenyl, 1E-pentenyl, 1 E-hexenyl, 1 E-heptenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

The term “fluoroalkyl” preferably covers straight-chain groups having aterminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. n is preferably =1 and m ispreferably from 1 to 6.

Through a suitable choice of the meanings of R⁰, R⁰′, R⁰″, X⁰ and X⁰′,the addressing times, the threshold voltage, the steepness of thetransmission characteristic lines, etc., can be modified in the desiredmanner. For example, 1E-alkenyl radicals, 3E-alkenyl radicals,2E-alkenyloxy radicals and the like generally result in shorteraddressing times, improved nematic tendencies and a higher ratio of theelastic constants k₃₃ (bend) and k₁₁ (splay) compared with alkyl oralkoxy radicals. 4-alkenyl radicals, 3-alkenyl radicals and the likegenerally give lower threshold voltages and smaller values of k₃₃/k₁₁compared with alkyl and alkoxy radicals.

A —CH₂CH₂— group generally results in higher values of k₃₃/k₁₁ comparedwith a single covalent bond. Higher values of k₃₃/k₁₁ facilitate, forexample, flatter transmission characteristic lines in TN cells with a90° twist (in order to achieve grey shades) and steeper transmissioncharacteristic lines in STN, SBE and OMI cells (greatermultiplexability), and vice versa.

The optimum mixing ratio of the compounds of the formulae I and II andIII to V depends substantially on the desired properties, on the choiceof the compounds of the formulae I to V, and on the choice of any othercomponents that may be present. Suitable mixing ratios within the rangegiven above can easily be determined from case to case.

The total amount of compounds of the formulae I to V in the mixturesaccording to the invention is not especially crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimising various properties. However, the observed effect on theaddressing times and the threshold voltage is generally greater, thehigher the total concentration of compounds of the formulae I to V.

Mixtures which, besides compounds of the formulae Ia and Ib and of theformula IIa, comprise compounds of the formula IV, in particular IVa orIVb, very particularly of the formula IVb, are distinguished by lowbirefringence and low threshold voltages.

The construction of the MLC display according to the invention frompolarizers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.The term “conventional construction” is broadly drawn here and alsocovers all derivatives and modifications of the MLC display, inparticular including matrix display elements based on poly-Si TFT or MIMand very particularly reflective displays.

A significant difference between the displays according to the inventionand the conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se. In general, thedesired amount of the components used in the lesser amount is dissolvedin the components making up the principal constituent, advantageously atelevated temperature. It is also possible to prepare the mixtures inother conventional manners, for example by using premixes, for examplemixtures of homologous compounds, or using so-called multibottlesystems, which typically consist of four corner mixtures which in eachcase in pairs differ in only one physical property.

The liquid-crystal media may also comprise further additives known tothe person skilled in the art and described in the literature. Forexample, 0-15%, preferably 0-10%, of pleochroic dyes or chiral dopantscan be added. The individual added compounds are employed inconcentrations of from 0.01 to 6% and preferably from 0.1 to 3%.However, the concentration data for the other constituents of theliquid-crystal mixtures, i.e. of the liquid-crystalline or mesogeniccompounds, are indicated here without taking into account theconcentration of these additives.

Above and below, unless explicitly stated otherwise:

percentages denote per cent by weight,

temperature data are indicated in ° C.,

temperature differences are indicated in differential degrees Celsius(degrees for short),

all physical properties are indicated for 20° C.,

the term “essential consist of” means consist to the extent of 50% ormore, preferably to the extent of 80% or more and particularlypreferably to the extent of 90% or more,

the term “compounds” without further restrictions, for clarificationusually written as compound(s), means a compound or preferably aplurality of compounds, and

denotes trans-1,4-cyclohexenylene.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) arestraight-chain alkyl radicals having n and m carbon atoms respectively,where n and m, independently of one another, are preferably an integerfrom 1 to 7. The coding in Table B is self-evident. In Table A, only theacronym for the parent structure is indicated. In individual cases, theacronym for the parent structure is followed, separated by a dash, by acode for the substituents R¹, R², L¹ and L²:

Code for R¹, R², L¹, L² R¹ R² L¹ L² Nm C_(n)H_(2n+1) C_(m)H_(2m+1) H HNom C_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) HH Nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H Nom C_(n)H_(2n+1) OC_(m)H_(2m+1) HH nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H N C_(n)H_(2n+1) CN H H nN.FC_(n)H_(2n+1) CN F H nN.F.F C_(n)H_(2n+1) CN F F NOF OC_(n)H_(2n+1) F HH NCl C_(n)H_(2n+1) Cl H H NCl.F C_(n)H_(2n+1) Cl F H NCl.F.FC_(n)H_(2n+1) Cl F F NF C_(n)H_(2n+1) F H H nF.F C_(n)H_(2n+1) F F HnF.F.F C_(n)H_(2n+1) F F F NCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₃.F C_(n)H_(2n+1) OCF₃ F H nOCF₃.F.FC_(n)H_(2n+1) OCF₃ F F nOCF₂ C_(n)H_(2n+1) OCHF₂ H H nOCF₂.FC_(n)H_(2n+1) OCHF₂ F H nOCF₂.F.F C_(n)H_(2n+1) OCHF₂ F F NSC_(n)H_(2n+1) NCS H H RVsN C_(r)H_(2r+1)-CH═CH-C_(s)H_(2s)- CN H H REsNC_(r)H_(2r+1)-O-C_(s)H_(2s)- CN H H NAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) HH NOCCF₂.F.F C_(n)H_(2n+1) OCH₂CF₂H F F

Preferred mixture components are shown in Tables A and B.

The mixtures according to the invention preferably comprise at least oneof the compounds of the formulae indicated in Table B.

TABLE A

CCH PCH

CCP BCH

ECCP CECP

EBCH CEB

TABLE B

CCZU-n-F

CDU-n-F

CCPC-nm

CPCC-nm

CBC-nm(F)

BCH-nm(F)

CH-nm

CPZU-n-F

CECU-n-F

CGU-n-F

CGU-n-OT

CC-n-V

CC-n-mV

CC-n-Vm

The entire of all applications, patents and publications, cited above,and of corresponding German application No. 10039379.9, filed Aug. 11,2000, is hereby incorporated by reference.

EXAMPLES

The following examples are intended to illustrate the invention withoutlimiting it. In these examples, All temperatures are given in degreesCelsius. m.p. denotes melting point, cl.p. clearing point. Δn denotesthe optical anisotropy (589 nm, 20° C.), Δε denotes the dielectricanisotropy (1 kHz, 20° C.), and the rotational viscosity γ₁ (mPa.s) wasdetermined at 20° C.

The physical properties of the liquid-crystal mixtures were determinedas described in “Physical Properties of Liquid Crystals” Ed. M. Becker,Merck KGaA, status Nov. 1997, unless explicitly stated otherwise.

V₁₀ denotes the voltage for 10% relative contrast (viewing angleperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) the switch-off time at a given operating voltage. Δn denotesthe optical anisotropy and n_(o) the ordinary refractive index, in eachcase at 539 nm, unless stated otherwise. Δε denotes the dielectricanisotropy (Δε=Δ_(∥)−ε_(⊥), where ε₈₁ denotes the dielectric constantparallel to the longitudinal molecular axes and ε_(⊥) denotes thedielectric constant perpendicular thereto). Δn is determined at 589 nmand 20° C. and Δε at 1 kHz and 20° C., unless expressly statedotherwise. The electro-optical data were measured in a TN cell (twist90°, pre-tilt angle 1°) at the 1st minimum (i.e. at a d·Δn value of 0.5)at 20° C., unless expressly stated otherwise.

The stability of the nematic phase at low temperatures was checked atfixed temperatures both in the flow viscometer and in sealed bottles inthe refrigerator. In addition, filled TN cells sealed with adhesive werestored in the refrigerator. The TN cells used corresponded to the cellsused for the electro-optical investigations. For each storage test at agiven temperature, at least five samples were used. The stable storagetime indicated was the time at which no change was finally observed inany of the samples. The rotational viscosity value for ZLI-4792 was 133mPa·s at 20° C. using the calibrated self-built rotational viscometer.

The following examples are intended to explain the invention withoutlimiting it. However, they give an overview of possible compositions ofthe media and of the achievable physical properties, in particular theircombinations.

Example 1

A 4-bottle system consisting of the four liquid-crystal mixtures A, B, Cand D and having the following compositions and properties was prepared.

Concentration/ % by weight Physical properties Mixture A Compoundabbreviation PCH-7F 3.0 Clearing point (T(N,I)) = 90° C. CCP-2F.F.F 12.0Smectic-nematic CCP-3F.F.F 13.0 transition (T(S,N)) < −40° C. CCP-5F.F.F8.0 n₀ (20° C., 589 nm) = 1.4740 CCP-3OCF2.F.F 13.0 Δn (20° C., 589 nm)= 0.0876 CCP-5OCF2.F.F 4.0 Δε (20° C., 1 kHz) = 9.4 CCP-2OCF3 7.0 ε_(⊥)(20° C., 1 kHz) = 3.7 CCP-3OCF3 8.0 γ₁ (20° C.) = 156 CCP-4OCF3 5.0 mPa· s CCP-5OCF3 8.0 d · Δn = 0.50 μm CGU-2-F 5.0 V₁₀ (20° C., 1 kHz) =1.42 V CGU-3-F 10.0 V₉₀ (20° C., 1 kHz) = 2.22 V ECCP-3F.F 5.0 Σ 100.0Mixture B Compound abbreviation PCH-7F 6.0 Clearing point (T(N,I)) = 91°C. CCP-2OCF2.F.F 14.0 Smectic-nematic CCP-2OCF3 8.0 transition (T(S,N))< −40° C. CCP-3OCF3 7.0 n₀ (20° C., 589 nm) = 1.4843 CCP-4OCF3 6.0 Δn(20° C., 589 nm) = 0.1103 CCP-5OCF3 8.0 Δε (20° C., 1 kHz) = 8.2 CGU-3-F12.0 ε_(⊥) (20° C., 1 kHz) = 3.9 CGU-5-F 8.0 γ₁ (20° C.) = 162 BCH-2F.F9.0 mPa · s BCH-3F.F 8.0 d · Δn = 0.50 μm BCH-5F.F 8.0 V₁₀ (20° C., 1kHz) = 1.46 V CBC-33F 3.0 V₉₀ (20° C., 1 kHz) = 2.27 V CBC-53F 3.0 Σ100.0 Mixture C Compound abbreviation PCH-7F 6.0 Clearing point (T(N,I))= 92° C. CCH-35 6.0 Smectic-nematic CCP-2OCF3 8.0 transition (T(S,N)) <−40° C. CCP-3OCF3 8.0 n₀ (20° C., 589 nm) = 1.4760 CCP-4OCF3 7.0 Δn (20°C., 589 nm) = 0.0860 CCP-5OCF3 8.0 Δε (20° C., 1 kHz) = 7.3 CCP-2F.F.F12.0 ε_(⊥) (20° C., 1 kHz) = 3.4 CCP-3F.F.F 11.0 γ₁ (20° C.) = 125CCP-5F.F.F 8.0 mPa · s CGU-3-F 5.0 d · Δn = 0.50 μm ECCP-3F.F 7.0 V₁₀(20° C., 1 kHz) = 1.68 V ECCP-SF.F 5.0 V₉₀ (20° C., 1 kHz) = 2.65 VBCH-3F.F 9.0 Σ 100.0 Mixture D Compound abbreviation PCH-7F 9.0 Clearingpoint (T(N,I)) = 92° C. CCP-2OCF3 8.0 Smectic-nematic CCP-3OCF3 7.0transition (T(S,N)) < −30° C. CCP-5OCF3 6.0 n₀ (20° C., 589 nm) = 1.4901CCP-2F.F.F 10.0 Δn (20° C., 589 nm) = 0.1114 ECCP-3F.F 10.0 Δε (20° C.,1 kHz) = 6.1 CGU-3-F 6.0 ε_(⊥) (20° C., 1 kHz) = 3.6 BCH-2F.F 9.0 γ₁(20° C.) = 136 BCH-3F.F 9.0 mPa · s BCH-5F.F 9.0 d · Δn = 0.50 μmBCH-32F 7.0 V₁₀ (20° C., 1 kHz) = 1.73 V BCH-52F 7.0 V₉₀ (20° C., 1 kHz)= 2.64 V CBC-33F 3.0 Σ 100.0

The TN cells achieved in this way are distinguished, in particular, bytheir low addressing voltage (threshold voltage and saturation voltage)in combination with excellent stability on storage at low temperatures.

The pairs of mixtures A-B and C-D each represent a 2-bottle system withvariable birefringence in the range from about 0.087 to 0.111, andmixture pairs A-C and B-D each represent a 2-bottle system with variablethreshold voltage in the range from about 1.4 V to 1.7 V. The fourmixtures A to D can thus be used as the corner mixtures of a 4-bottlesystem which covers these ranges.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A liquid-crystalline medium of positivedielectric anisotropy, which comprises: one or more compounds of theformula I:

in which R¹ is an alkyl or alkenyl radical having 1 or 2 to 7 carbonatoms, and X¹ is F, OCF₃ or OCHF₂; one or more compounds of the formulaII

in which R² is an alkyl or alkenyl radical having 1 or 2 to 7 carbonatoms, and X² is F, OCF₃ or OCHF₂; one or more compound(s) of theformulae IIIb or IIIg

which R³ is an alkyl of 1 to 7 carbon atoms or alkenyl radical of 2 to 7carbon atoms; one or more compound(s) of that formula IV

in which R⁴ is an alkyl or alkenyl radical having 1 to 7 carbon atoms oralkenyl radical having 2 to 7 carbon atoms, X⁴ is F, Cl, and k is 0 or1; and one or more compounds of the formula V

in which

are each, independently of one another,

R⁵¹ and R⁵² are each, independently of one another, an alkyl or alkoxyradical having 1 to 7 carbon atoms or alkenyl radical having 2 to 7carbon atoms, and n and m are each, independently of one another, 0 or1; wherein the medium exhibits a nematic phase at least down to −20° C.and at least above 75° C., birefringe value of ≦0.090 or ≧0.100, and arotational viscosity, γ₁, at 20° C., of less than 160 mPa·s.
 2. Themedium according to claim 1, which further comprises one or morecompounds of the formula III, which are not of formula IIIb or IIIg inclaim 1:

in which R³ is an alkyl radical having 1 to 7 carbon atoms or alkenylradical having 2 to 7 carbon atoms, Z³² and, if present, Z³¹ are each,independently of one another, —CH₂—CH₂—, —CH═CH—or a single bond,

X³ is F, OCF₃ or OCHF₂, and r is 0 or
 1. 3. A medium according to claim1, wherein the proportion of compounds of the formula I in the medium asa whole is at least 5% by weight.
 4. A medium according to claim 2,wherein the proportion of compounds of the formulae II, IIIb, IIIg, III,IV and V together in the medium as a whole is from 40% to 90% by weight.5. A multibottle liquid-crystal system which comprises a mediumaccording to claim
 1. 6. An electro-optical device which comprises aliquid-crystalline medium of claim
 1. 7. A medium according to claim 2,which consists essentially of compounds of the formulae I, II, IIIb,IIIg, III, IV and V.
 8. A medium according to claim 1, which exhibits anematic phase at least down to −30° C. and at least above 80° C., abirefringence value of ≦0.085 or ≧0.105, and a rotational viscosity, γ₁,at 20° C., of less than 130 mPa·s.
 9. A medium according to claim 2which comprises a concentration of 3-65% compounds of the formula I,3-40% of compounds of the formula II, 2-50% of compounds of the formulaeIIIb, IIIg and III, 10-50% of compounds of the formula IV and 30% orless of compounds of the formula V.
 10. A medium according to claim 2,which comprises more than 50% of compounds of the formulae I, II, IIIb,IIIg, III, IV and V.
 11. A medium according to claim 2, which comprisesmore than 50% of compounds of the formulae I, II, IIIb, IIIg, III, IVand V.
 12. A medium according to claim 1, wherein, in formula IV, X⁴ isF.
 13. A medium according to claim 1, which comprises a compound of theformula IV wherein k=0.
 14. A medium according to claim 1, whichexhibits a rotational viscosity, γ₁ at 20° C., of less than 130 mPa·s.15. A medium according to claim 1, which exhibits a birefringence valueof ≦0.080 or ≧0.110.
 16. A medium according to claim 14, which exhibitsa birefringence value of ≦0.080 or ≧0.110.
 17. A medium according toclaim 1, wherein the medium comprises at least one compound of theformula IIIg.
 18. A medium according to claim 1, wherein the mediumcomprises at least one compound of the formula I wherein X¹ is F.